Zoned air heater



June 11, 1957 R. s. COOPER z'oNED AIR HEATER Flled Dec 30y 1954 I I l I I l I l l 1 v l a I I I l.

' ATTORNEY ZONED AIR HEATER Roland S. Cooper, River Forest, lll., assgnor to The Air Preheater Corporation, New York, N. Y., a corporation of New York Application December 30, 1954, Serial No. 478,843

9 Claims. (Cl. 122-1) The present invention relates to heat exchangers and particularly to a gas recirculation system as used in conjunction with a rotary regenerative heat exchanger having primary and secondary air heating sections.

In a rotary regenerative heat exchanger a cylindrical rotor has compartments carrying metallic heat transfer plates which as the rotor turns are rst exposed to heating gases and then disposed in an air passage to impart the absorbed heat to cooler air passing therethrough. The rotor is surrounded by a housing having end or sector plates formed with openings to provide passageways for the How of hot gas and cooler air therethrough.

`The rotor of a rotary regenerative heat exchanger may have a primary and secondary air heating section by providing it with two independent zones through which air and gas may be circulated in amounts required to obtain separate streams of air at the different temperatures desired, as for example, primary and secondary air for the combustion of fuel. An air preheater of this type is disclosed in United States Patent 2,347,857, granted on May 2, 1944, to Joseph Waitkus and constitutes the basic apparatus in which recirculation system of the present invention is embodied.

Ordinarily a boiler having an economizer section is arranged so that all the gaseous products of combustion leaving the economizer pass through the heat exchanger (e'. g. air preheater) to an induced draft fan. On the air side of the heat exchanger air from a forced draft fan passes through the heat exchanger where it absorbs heat from the gas before passing on to the point of use. The usual gas temperatures from the economizer range from 650 degrees F. to 750 degrees F. and in passing through the heat exchanger they are reduced to from 275 degrees F. to 325 degrees F., the heat from the gas being given up to secondary air which is preheated to temperatures ranging from 500 degrees F. to 600 degrees F. Very often wet coal in the mills of a coal fired `boiler requires a somewhat higher primary air temperature of from 650 degrees F. to 750 degrees F. in order to properly dry the coal and maintain its ineness so that it will burn in the correct zone within the furnace. Consequently, a preheater capable of simultaneously providing several degrees of preheated air is essential to such a system and provides the basis of this invention.

Since primary air temperatures of from 650 degrees F. to 750 degrees F. are required, high temperature gas is taken ahead of the economizer or from some similar zone of the boiler which must be from 800 degrees F. to 1000 degrees F. in order to provide sulcient temperature head within the air preheater to effect the desired transfer of heat. If the thermal efliciency of the primary air section of the rotor is substantially the same as the thermal eticiency of the secondary air section, the gas leaving the primary air section will be from 400 degrees F. to 600 degrees F. as compared with 275 degrees F. to 325 degrees F. for the gases leaving the secondary air section. If both streams of gas were then mixed, the average gas temperature leaving the preheater would be higher Patented June 11, 1957 than necessary resulting in a loss of boiler efficiency. Furthermore, in taking high temperature gas from ahead of the economizer in the amount of approximately 20% of the total gas, the mass flow over the economizer would be decreased and the performance of the economizer affected. The effect in general that could be expected would be less water temperature rise, which would in turn require the burningof more fuel or a loss in boiler eiciency.

It therefore becomes a general objective of this invention to eiciently utilize the heat content of the higher temperature exhaust gases being discharged from a primary air zone of a temperature zoned air preheater. An-

other objective of this invention is to provide an arrangement whereby gas from the primary section of a temperature zoned preheater may be returned to the boiler in advance of the economizer to re-establish the mass flow through the economizer.

The invention may be best understood upon consideration of the following detailed description of an illustrative embodiment thereof when read in conjunction with the following drawing in which:

Figure l is a schematic illustration in side elevation of a boiler plant embodying the present invention.

Figure 2 is a fragmentary plan View of the rotor of the rotary air heater.

Referring more particularly to Figure 1, the plant shown comprises a boiler 10 which may be of -any desired type and which is illustrated as provided with a furnace 12 suitable for the combustion of powdered coal as fuel. The products of combustion, after leaving the furnace 12 pass over a superheater 14 and then over an economizer 16 before passing into the duct 20 leading to the gas side of a temperature zoned air preheater 26.

The air preheater diagrammatically illustrated is a temperature zoned rotary regenerative air preheater, characterized by a rotor 21 having separate concentrically arranged inner and outer temperature zones 24, 25, through either of which air and gas may be circulated in the amounts required to obtain separate streams of air at different temperatures desired.

The amount of heat imparted to the volumes of air owing through the two separate heat zones of the preheater may be varied by providing different amounts of heat transfer surface in the primary and secondary sections 24 and 2S.

Dampers 32 and 34 in ducts connected to the gas passage adjacent the economizer and upstream thereof regulate the volume of high temperature gas to be directed to and from the primary section 24 of the air preheater. A damper 36 adjacent the primary gas outlet may be adjusted to regulate the amount of exhaust gas from the primary section 24 Which is to be recirculated by fan 40 back through duct 42 to the economizer 16.

A damper 38 in the inlet duct 44 to the primary section 24 may be opened so as to interconnect the duct 44 and the gas inlet 20 so as to supply gas from the economizer to both the primary and secondary sections in order that they operate as a unit at a constant temperature rather than as individual heating sections at different temperatures.

By adjusting the dampers as disclosed the amount of gas passing through the air preheater may be so distributed to the primary and secondary sections of the rotor and the amount of gas recirculated to the economizer may be so controlled that the temperature of heating elements within the preheater may be kept at any desired temperature in order to transmit a particular amount of heat to the fluid being supplied by forced draft fan 41 to the primary and secondary air sections 24 and 25 of rotary air preheaterv 26. By way of example, air heated in primary air section 24 leaves via duct 45 at a temperature ranging from 650 degrees F. to 750 degrees F., while preheated air from secondary air section 25 is ported `through a duct 47 `at a somewhat lower temperature ranging from 500 degrees F. to 690 degrees F.

After the `hot gases from duct 44 pass throughthe primary gas section`24, into'outlet duct 46 theystill areat a considerably higher temperature than 4are the gases that have flowed over `the cconomizer 16 `and then `passed` through the secondary gas section 25 into duct 48. By comparison, the temperature of the gases exhausting from the primary section 24 into outlet duct 4f range from 40) degrees F. to 600 degrees F. while gases exhausting into duct 48 from secondary section 25 range from 27,5 Vdegrees F. to 325 degrees F. Since the gases from the primary section still carry a portion of their original heat Content and are at a much higher temperature than are the gases being exhausted from the secondary section, provision has been made to return thetnrvia the duct 42 to a point in advance of the economizer where `they are combined with hot gases from the `boiler to maintain a high mass how through the economizer and secondary section 25, of the preheater 26 where they give up still more heat before being exhausted via an induced draft fan (not shown) to the atmosphere.`

What is claimed is: l

l. ln a boiler plant having a fuel burning furance, an eeonomizer section with a hot flue-gas outlet, a rotary regenerative heat exchange apparatus having spaced passages for the flow of hot flue gas and relatively cooler air to be heated, inlet and outlet ducts directing hot flue gas and cooler air to be heated to and from the spaced passages in the heat exchange apparatus, a rotor in said heat exchange apparatus containing heatfexchange material adapted to be rotated through the gas passage to absorb heat from hot gas and then into the air passage to impart heat to air passing therethrough, and an annular partition positioned intermediate the outer edge of the rotor and its axis et rotation dividing it into inner and outer annular zones; bale means disposed in the gas inlet duct aligned with jsaid annular partition; other baille means in the gas outlet duet providing inner and outer zones in substantial alignment with corresponding zones of said rotor; a fluid passageway supplying high temperature gas to the outer section of said gas inlet duct from a bleed point in said boiler plant in advance of said cconomizer; and a passageway connecting the outer zone of said gas outlet duct to a return point between said bleed point and the economizer whereby spent gas from the outer annular zone ofy the rotor is circulated through the economizer and the inner annular zone of said rotor before being exhausted to the atmosphere` 2. A boiler plant as defined in claim l wherein damper means across the outer zone of the gas outlet duct regulates return ow of cooled gas to the boiler whereby ternpcrature of the gas flowing to the secondary zone of the temperature zoned air preheater maybe controlled by regulation of said damper.`

3. A boiler plant having a fuel burning furnace, an ecenomizer section with a hot flue `gas outlet, a rotary regenerative heat exchange apparatus having spaced passages for the iiow of hot flue gas and relatively cooler air to be heated, inlet and outlet ducts directing hot flue gas and cooler air to be heated to and from the spaced` passages in the heat exchange apparatus, a rotor in said heat exchange apparatus containing heat exchange material adapted to be moved first through the gas passage to absorb heat from the hot gas and then into the air passage `to impart heat to air passing therethrough7 and an annular partition positioned intermediate the periphery of the rotor and its axis of rotation dividing it into outer primary and inner `secondary sections; bale means dis posed in the gas inlet duct in substantial alignment with said annular partition subdividing the inlet duct into innerV preheater is returned to the boiler to maintain the mass flow of the hot gas flowing through the economizer.

4. A boiler plant as defined in claim 3 wherein damper means regulates flow of hotgas` from the` boiler bleed point to the outer zone of the rotor.

5. A boiler plant as defined in `claim 4 wherein the damper means is arranged to move between a port in the gas inlet duct and said passageway whereby closure of said damper means across the passageway opens the outer zone of the rotor to fluid ow from the economizer out let. i'

6. The method of extracting heat from the hot gases produced in a `fuel burning boiler furnace including an economizer section which comprises the steps of bleeding a quantity of hot gas from the boiler, directing the hot gas through the primary zone of a temperature zone air preheater, returning partially `cooled gas from the outlet of said primary zone to the boiler in advance of said economizer, and combining` the partially cooled gas with uncooled gas before passing it through the economizer whereby the mass flow of the gas through the economizer is maintained constant to avoid loss in boiler eiciency.

7. In a plantproducing hot gases and having a gas passage containing aheat exchanger with a hot gas out let7 a rotary regenerative air heater with a rotor containing heat exchange material adapted to absorb heat from hot gas, `inlet and outlet ducts for directing hot gas from said exchanger outlet through said rotor, and a circular partition positioned intermediate the outer edge `of the rotor and its axis of rotation dividing it into inner and outer annular zones; baille means disposed in the gas inlet and outlet ducts for said air heater located in alignment with said circular partition in the rotor for providing separate sections in said gas inlet and outlet duets in substantial alignment with the inner and outer zones of said rotor; a duct connecting the gas outlet of said heat exchanger with one section of the gas inlet forsaid `air heater; a duct connecting the other section of `the gas inlet duct for said air preheater with said gas passage at a point upstream of said heat exchanger; and a duct connecting the section of the outlet duct corresponding to said other section of said inlet duct with said plant gas passage at a point between said heat exchanger and the connection of said first duct to said plant gas passage.

8. in a plant producing hot gases and having a gas passage containing a heat exchanger with a hot gas outr let, a rotary regenerative air heater with a rotor containing heat exchange material adapted to absorb heat from hot gas, inlet and outlet ducts for directing hot gas from said exchanger outlet through said rotor and a circular partition positioned intermediate the outer edge of the rotor and its axis of rotation dividing it into inner and outer annular zones; batilemeans disposed in the gas inlet and outlet ducts for said air heater located in alignment with said annular partition in the rotor for providing separate sections in said gas inlet and outlet ducts in substantial alignment with the inner and outer zones of said rotor; a duct connecting the gas outlet of said heat exchanger with one section of the gas inlet `for said air heater; a duct connecting the other section of the gas inlet duct for said air preheater with the plant gas pas sage at a point upstream of said heat exchanger; and a duct connecting the section of the outlet duct that correspends to said other section of said inlet duct with said plant gas passage at a point between said heat exchanger and the connection of said first duct to said gas passage; and a damper controlledport in the bale means in said gas inlet duct of said air heater for placing the two sections thereof in communication with each other.

9. In a plant producing hot gases and having a gas passage containing a heat exchanger with a hot ue gas outlet, an air heater having passages for the flow of hot v gas and relatively cooler air to be heated, inlet and outlet ducts directing hot gas and cooler air to be heated to and from the passages in the air heater, and partition means on the air side of said heat exchanger for dividing it into separate passages for heating separate streams of air; bafe means disposed in the gas inlet and outlet ducts for said air heater located in alignment with said partition means for providing separate sections in said gas inlet and outlet ducts in substantial alignment with the separate passages of said rotor; a duct connecting the gas outlet of the heat exchanger with one section of the gas inlet for said air heater; a duct connecting the other section of the gas inlet duct for said air preheater with said plant gas passage at a point upstream of said heat exchanger; and a duct connecting the section of the oulet duct that corresponds to said other section of said inlet duct with said plant gas passage at a point upstream of said heat exchanger between it and the point of connection of said first duct to said plant gas passage.

Waitkus May 2, 1944 Yerrick Aug. 19, 1952 

